There is no solution to this problem.
Period.
For every situation that you attempt to prevent, there is a different situation that can arise as a result of that solution.
For example, if the emergency code opened the door, and the PF was unable to prevent that, then in the case of the captain who went "nuts", the FO could not prevent him from entering the FD.
If we used a CC to stand inside the door and wait until the PNF returned, then verified them through the peep hole, we have the possibility that the PNF has been incapacitated by a hijacker and is standing there with a gun to his back.
If you have a completely isolated FD (moving the locked door back), then what do you do if the crew become incapacitated (bad fish dinner).
Whatever "solution" you come up with, there will always be another situation that is not covered by that solution.
This is not something that can be solved by putting in more or different technology. It is a symptom of our society, and we need to address the problems in society.
But that's a far greater challenge.
(all of the above is based on accepting the - to my mind extremely premature - conclusions of the investigations so far) OR if you don't accept those conclusions, then at least considering the possibility of a pilot who "loses it" and decides to either lash out against the other pilot, or prove to his "ex" that they will miss him, or thinks that it will ensure something or other in the life hereafter. We have to remember that the person who wants to destroy the pilot, could be on either side of the door.
So we can only have a partial solution, one that solves a percentage of the problems, and then we have to hope that we've chosen the right portion.
And, as some have attempted to say that this policy is worse than the opposite, because "more have died" from this policy than it has saved, then we should recognise that there is absolutely no way to count the number that have been saved.